Telephone circuit



New. 8, 1949 A. LESTI TELEPHONE CIRCUIT 2 Sheets-'Sheet l Filed Jan. 29,1945 Nov. 8, 1949 A. LES-rl 2,487,455

TELEPHONE CIRCUIT Filed Jan. 29, l1.945 f 2 sheetssneet 2 M/CROPHO/VfHMPL /F/[R 3 /7- ,9+ 5- 5+ 6 6 4 afP/L/o/vf 5 J Pfc. wf

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BY 29m/@@1122 Patented Nov. 8, 1949 Arnold Lesti, Rochester, N. Y.,assigner to Stromberg-Carlson Company, Rochester, N. Y., a corporationof ew York Application January 29, 194,5, Serial No. 575,096

9 Claims.

' This invention relates to telephone circuits for a microphone anda'receiver such as are utilized at'subscribers telephone substationsland the like.

rThe carbon microphone or transmitter has beenA almost'unversally usedin subscribers telephone Sets because, in addition to modulating currentin accordance with sound, it also possesses lthe unique property ofacting as an amplifier. However, the carbon microphone is subjecttohighly undesirable defects, such as carbon noise, packing, deteriorationwith age, positional defects, non-linear `distortion, and distortionwhich results in modulation products when two frequencies are impressedthereon. Extensive research on the carbon type of microphone has failedVto overcome these defects. It is therefore the purpose of thel presentinvention to eliminate the carbon microphone in substation circuits andto substitute therefor a high quality microphone which may be any one ofseveral kinds, commonly designated electro-magnetic, moving coil,crystal, condenser or velocity types. However, such high qualitymicrophones have no inherent amplification and consequently do notdeliver sufhcient output to actuate to the desired level, the receiveratA a distant station. It therefore becomes necessary to associate anamplifier with the microphone, preferably at the substation, so that theamplified output of the microphone will be at the desired` level.vHowever, the use of such an amplierA introduces the problem of providinga power supplyl source without disturbing hum.

Furthermore, substation circuits of the type under consideration arepreferably arranged to have anti-side-tone characteristics and ittherefore becomes a real problem to provide an anti-sidetone networkinto which the output of the amplifier'can be efficiently introduced.

Where an amplifier is provided at a subscribers substation, it is highlydesirablethat the vacuum tube or tubes in the amplifier be operated onlywhen this substation is in use. However, vacuum tubes whosel cathodesare heated from a rectified andfiltered alternating current sourcerequire a substantial interval of time before their cathodes are heatedto the degree where there is ample electron emission therefrom to renderthese tubes operative. The required heating'interval necessary to renderthe cathodes of such vacuum tubes effective, is longer than the timeinterval between lthe start of the lifting of the handset from thedesk-stand and the placing of the transmitter'up to the mouth.Consequently, if conventional vacuum tube amplifiers are used, it is'possible for the subscriber to speak into the transmitter before theamplifier is operative, with the result that a portion of theconversation can be 10st,

The main feature of the present invention therefore relates to a novelarrangement for effecting high speed heating of the cathodeor cathodesof the vacuum tube or tubes inv anamplien; so lthat the amplifier canbel used at va subscriberssubstation,and will be operative'before thesubscriber starts talking.

A further -feature of the invention relates to an arrangementforderiving the vpower 'for a telephoiieisubstation from the commerciallighting circuit available at the substation.

yAn `additional 'feature' of the invention relates to a telephonesubstation including a high quality microphone and anr amplifiertherefor ypowered from: the 'commercial lighting circuit, togetherwith'a normally ineffective standby carbon microphone automaticallyrendered yeffective and poweredover the telephone line from'thecentralexchange, whenever the lighting source fails.

' Other featuresfand advantages of the invention will appear from thedetailed description and claims-when taken with the' drawings in which;l,

Figpil is a diagrammatic showing of a novel substation circuitarrangement 'in accordance with thepresent invention;' Fig. 2'is'adiagrammatic showing of a modified 'form of rectifier which can besubstituted for the gas tube rectifiers in Fig; l;

' Fig. 3` isa simplified diagram of a portion of the filter network ofFig. l;

Fig. 4 is a diagram useful in describing the connection of the amplifierinto the anti-sidetonecircuit arrangement of Fig. l; and

Fig. 5 illustrates a simplified showing of a circuit whcrein a highlquality microphone and amplifier are powered from the commerciallighting current source at the substation` and wherein there is provideda carbon microphone in standby 'relation ready to be cut into serviceand to be powered over the telephone line from the battery at' theexchange when the commercial lighting current source fails.

'Referring especially to Fig.. 1, the substation telephone set thereindisclosed includes a microphone 5 of al high quality type such as aVcrystal microphone, or if coupled to the amplifier in a manner wellknown to the art an electromag- -netic'microphone, a moving coilmicrophone, a

traveling springs.

is preferably of the crystal type which is especially suitable becauseof its small size. The substation set also includes a receiver 6preferably of the watch case type so that the microphone E and thereceiver 6 can be mounted in the handle of the usual hand set 1, morecommonly referred to as the French type of telephone. The substationA.set also includes a conventional ringer` 8 and equipped with or with.-out a dial impulse sender Si, as occasion demands. There is alsoincluded in the substation set `a lso-called hook-switch actuated to itsalternate positions in the conventional marmer bythe removal from andthe replacement respectively, ofl the hand set "l on the cradle yof thetelephone stand (not shown). The hook-switch comprises sets of contactsprings iO-ll, I2-I3, I4--l5V and Iii-H. The operation of travelingcontact springs of each set is delayed by an air dashpot comprising afixed cylinder I8 and a plunger i9 movable therein under the inuence ofthese Contacts Iii-H close first to complete an alternating currentpower circuit which, in combination with-a suitable rec- .tier unit tobe described, conducts pulsating .rectified current through thefilaments of the amplifier tubes to be referred to,.and this heats `upthese tubes quickly. The air dash-pot increasesthe time from the closureof spring contacts ltl-Il to the closure of spring contactsl 14..- ofthe hook-switch. During this interval the rectified alternating currenthaslample time to heat up the filaments ofthe tubes. When contacts llil5close, they thereby connect certain" capacitors across all filtercircuits for hum `elimina-tion f as will be described. This occursfaster-,than the time required to remove the hand Vsetfroi'n thehook-switch and place the receiver minimizes the 'duration of momentarynoise ,onl

the line. 7

i Since the present substation. arrangement is intended to be used wherea high signal output is desired, it is essential that the response ofthev microphone -5 be amplified. This is effectedby a two-stageamplifier comprising the, electron tubes 2li and 26, which areof thedirectly heated pentode type, and may be of the type now commonly soldunder the code numbers 1L4 and 3Q4,

respectively.- The tube is providedwith electrodes including afilamentary cathode 2|, a control grid 22, a screen grid 24, asuppressor grid 23 and an anode or plate 25. Likewise, the tube 26 isprovided with corresponding electrodes designated y27, 28, '29, 3l) and3i. The amplifier tubes 20 `and -213 are resistancefcoupled by thecapacitor 32 and the resistors 61 and 33 the last-named of which isconnected potentiometer fashion to the `,control grid 28l to serve as,a` volume control.'

These amplifier tubes are powered by rectified and filtered'currentderived from the alternating current leads 34 and 35 of thecommercial'source wof electric lighting current available at thetelephonev subscribers residence where they substa-v tion setis-located;i l

Since the amplifier tubes are lighted only duringthe time that thesubset is in use, these tubes as well asv the rectiflers must becomeeffective in. a period -o'f 'about one-quarter .of a second which isshorter than the interval between the lifting of the telephone handsetfrom its cradle and the time when the subscriber first speaks into themicrophone. The use of indirectly heated cathode tubes is thereforeprecluded, since they require up to twenty seconds to heat up from acold start. Instead, directly heated fllamentary type tubes-arenecessary, but this alone is not enough. A'circuit with these lattertubes and a resistance capacitor filter adequate to lter the hum, takesabout 1.1 seconds to warmup. The long time-constant of the filter,together with some thermal inertia of the filaments slows the action. Inaccordance with the present invention, there is provided a high speedcircuit, which reduces the warming-up time to about 1% second. In thiscircuit, cold-cathode 'y gas rectifier tubes 36 and 31 are used toinsure The latter prevent noise in yconductor t5 throughresistors S6 and61 to the fast starting. However, a single selenium half wave rectifier38 may be substituted for the gas rectifier tubes as shown in Fig. 2.Capacitors 'iti and 'H merge into one unit, if this substitution ismade. Two separate filter networks F-,l and SEL-2 for the filament andanode supplies respectively, are provided. The filter F--I includes theresistors 39 and 4i) in series in the positive conductors 6I and 62 ofthe filament circuit to be described. This filter also includescapacitors 4l and 42, capacitor 4I having one terminal thereof connectedto the common terminals of resistors 39 and 4D. Capacitor 42 has one ofits terminals connected to conductor 6,2. The plate filter F-Z includesthe resistors 49 and 5t in series with the plate supply conductor 5i andthe capacitors 53 and 54. The capacitors 4|, 42 and 53, 54 have theirnegative leads 55 and 'l5 connected to the hook-switch contact spring l5and must be extended through springs itl-I5 to complete the circuit tothe common negative return lead 57. These capacitors are in an openedcircuit when the amplier is not in use, and also during the interval oftime between the closing of contacts lll-l l of the hook- .switch andthe closing of contacts lli-I5, which interval begins substantially whenthe hand set is lifted by the subscriber. During this interval, therectifiers 3S and 3i are functioning so that pulsating current iiowsthrough the filament circuit to heat up the filaments ofV the amplifiertubes 20 and 26. This filament circuit may be traced from power lead 34,conductor A59, hookswitch contacts ifi-ll, conductor 6B, throughrectifier 3?, conductor 6l, filter resistors v39 and 4Q, conductor t2,maments 2l and 2|, conductors 63 and 5T, to the power lead 35. The platesupply for the amplifier tubes extends from the power lead 34,conductors 59 and 6i! through the rectifier 36, conductor 5l, throughthe resistors 49 and 5d in series, to the junction 64. From thisjunction, the circuit to amplifier 2l) extends over screen. grid 24anode 25 respectively of tube 20. From junction 64, the conductor 68extends to the screen grid of tube 26 and through coil 69,` to the anode3l of tube 26,

Condensers 10 and 'il respectively connect the conductors 6l and 5lfromthe rectiiiers 31 and 365 to conductors 'l2 and 5l, leading to thepower lead 35,

`,thereby providing a return circuit for the alter- `and 'it dueto thetime required to charge them which tous etrierwis teiqiiird. Theintertai estime jtween the @rising @i spariscontacts iti-"ii anali- 15jofthe rigor-switch is increased by the. air dash-pot connected to thetraveling springs, but this;V sio'vw intervaiis shorter than the longestpermissible heating- 11p time. lrhs slew interi/.ai aiiows enough.puisations, which are separated by 1/f'io Second, .to 1iiow fhroughthefilaments o f the ampliertubesto insure their heatinglup.Sulo1sequently, when "slhiitingeirct o 'f the capacitbrs ai aridi-2aiises a momentary drop inv filament vcurrentI which, ecaus'e ofthermalinertiacf the ilarnents 2| and 21, is insufficient tolowerthetemperature of these filaments sufficiently to V stop electronicemission. ,iOther factors outlined' hereinafter, are effective whichcontribute to an even faster action.

, During theinterval oi spring travel, thecapaciicml ,which isfoniletedto theiunctiou f the resistors 39 and 40 isin series withthecapacitor as illustratedin Fig. 3. Consequently,

during such an interval, the resistor 4|] isshurited by a capacita ncecombination in series v 4I, 4 2 and addition .to the current flowingthrough the resistor 4|.'l there is at first also a current surgethrough the capacitors 41, 42., These cornbineilcurrentsi'lowin thefilaments of the ampliuer tubes, andhence decrease the interval requiredtoraise the temperature of these -:lilarnents to ,the value whereelectron emission occurs. Moreover, both these lament circuitcapacitors, 4|, 4 2 andthep1ate circuit capacitors 5 3, m54 remaincharged after tl'iehaiids.et l .iS lgplacedonthe i hooksys/itch,becausethe common return lead 5 6 oi ,these capaciterais open at thattime. Thestoredcharge.speeds up subsequent starting times yeven further,for a considerable periodaiter the handset has been used a iirst time.In the first operation, from' a cold uncharged conditiongachargeiisdeveloped on the capacitors which minimizes the shunting action whencontacts 4 and 'I5 of the hookswitch close. The warming-up of thefilaments from ac old first startto a point where, the 'speech istransmitted is about T-.Second Repeatedon and off operation of thehook-switch to flash the operator charges the capacitors 4I., 42 andpre-.-

vents surges. The filaments are thus protected and the circuit isfool-proof. The surgethrough the ,bavio'r 4| and ileso-compensatesiarthe Alowered starting ,voltage gluel to the ncrease d 6,0 lcycleirripedancel the rectiiierJcircuit when the capacitors are openedbyspringsL l4 -fl 5 of the .,hdokrw-tlcu-, This. increased impedance lowrs the starting ,surge through the rectifier t1i b es36;. 31 and insuresthat they will have The above .high speed circuit requires specialprecautionsto insure duiet operationbofthe ampliier. Referring to Fig.l, the, capacitor i 4 2 must filter zthe, filament circuithtoythehpointyvh're noA can be h'eard. 'Ihe return lead. of. this capacito'rA is,connectedby conductor: 1 5

directly to spring l5. of the lloo'kJ-s v vitch ar 1d is separate fromthereturn lead,5 6. of the other capacitors which isalso connected tospring 15.

to such a 10W iev'ei that it is ot'noticed 6r use Tire crystalmicrophone s iii themndst 1 is completely, shielded and is connectedtothe volt,- age ampiinertubezo by a shieiuedcableietith shieldedcoupling 11. The shielding grmrtiiiscabie must be closely meshed.A tT hemicrophone is connected directly to the control grid 22 of the tube 20,with the shielding connected to the cathode 2| thereof, to provide areturn path for the micrqf phone current and to insure quietriefss`. yThe tube- 20 is a pentode voltage amplifier and is resistance coupled tothe pentodepower tube 26....with volum'e control potentiometer aspreviously meritioned. A slight amount of inverse feed-back on tube 26and its input network is provided by the resistor {0 0, The plate of theltube .2.615, oo nnected to the primary Winding 69 of the iii'dctioncoil designated as '|9 This 6011 is analogous t"o the local battery'anti-sid-tone coil of conventional substation circuits, but in thiscase the carbon transmitter thereof is replaced ^by Ythe tube 26; andthe coil impedancesar suitably altered as will be described. The,secondary windings 82 and 83 as well as the resistors and 81 togetherwith capacitor 88 are the Qihr impedancesofthe antiside-,ton'e netti/Ork hereinafter discussed. In the hook-switch', Sp' iig terminalsI'2--I3 close the telephone linecircuit through to the induction coil,while. the coil designated 80 is a choke which providesia low resistancedirect current .pathfor central office supervisory purposes. This latterarrangement, together with the opening of the receiver circuit atsprings Iii-l1 of the hook-switch; and the connections to the dialsender 9 arey conventional. Springs III-II of rthe hook-switch controlthe power circuit as mentioned and close first Awhen the handset istaken from its cradle. The hous- This reduces thevcommon resistancebetween the l quiet capacitor 42 andthe other capacitors which areconnected to points', in the circuitiny which' the. voltage pulsationsare high,v and hencewould i'riduj Vpillsatina .voltages in thecapaci11011. 4,2.

ing, (notl shown) of theinstrument is not` connected directly to anypoint of the circ t; but the .005 mf. capacitor 8l is connected tthereturn conductor 51 of vthe power circuit. The housing is safe and.noelectrical shock can' be obtained from it. The capacitor 8l eliminatesine noise. Conductor SI1 and terminal 9i provide a convenientarrangement for testing the filament voltage.

Amplifier tubes, such as 26, do not necessarily have their output -plateload impedance equal to their plate impedance. For example, thetubeZB'should Work into a load often thousand ohms for 'maximum undistortedpower output, but its plate impedance is rated at one hundredtwentythousand ohms. In the conventional aritifsdetone substation, theinduction coil used thereinis designed in accordance with welll-knownformulas since the load impedance ofthe telephonetransmitter ormicrophone is equal toits internal irnpedance. However, the fact thatthe plate load of tube 26 is ten thousand ohms while its plate impedance(internal impedance) is one hundred twenty thousand ohms, made it`appear impossible to substitute apower Iamplifier tube, such as 26, forthe usual transmitter in, $11.6. conventional anti-sidertone Vnetworkand stillretaih ,.a maximum output network. For desirable operation it afirst step. This iixes Equations 4 and 1.

is important that the tube 26 should work into its optimum load (10,000ohms), the subset circuit should be of the anti-side-tone type, andreceiver circuit efciency should be as high as possible. It was onlyafter extended study and experimentation that the present anti-side-tonenetwork (Fig. l) was discovered. The desirable operation outlined aboveis achieved in the antiside-tone network of this invention, as will beseen from the following analysis of the circuit diagram of Fig. 4 whichis a simplified showing of the anti-side-tone network of Fig. 1. In thiscircuit the condition of side-tone balance is achieved in the firstconjugacy condition given by the relation.

g=Z-N (1) The second conjugacy condition ordinarily existing inanti-side-tone circuits, namely, that an E. M. F. acting in ZL producesno current in ZN holds approximately when N/ZCZILZR zT-2 2 2) Maximumpower is delivered from ZT when ZCzDJfa/zcz' D=ZL+ZN (3) ZA ZT The powerdistribution ratio from ZT to ZL has the reciprocal property and equalsthat from ZL to ZT; that is, with the source in ZT the ratio of powerdelivered to ZL to the total that can be delivered by ZT is equal to theratio of the power received by ZT, with the source at ZL, to thetotalfpower that can be delivered by ZL.

The power distribution ratio Y is equal to the ration of power receivedby ZT to the power received by ZR with the source in the line. In theabove circuit when the condition given by Equa tion l is satised Y isgiven by In the present arrangement the transmitting output can beincreased by the volume control, that is, by increasing theamplification. This feature can be `taken advantage of to increase thereceiving eiciency, as compared to conventional sets, even though thereis no amplification for receiving. The ratio Y can be made small so thatthe power transmitted to the line, with an E. M. F. in the transmitter,will go down, but the power in the receiver, with an E. M. F. in theline, will increase, and by utilizing amplication to step uptransmitting emciency, both high transmitting and receiving e-licienciesmay be attained.

The choice of Y for the present arrangement is Then Equation 3 can beapplied to connect the tube to its required load of 10,000 ohms by usingthis value of rated load impedance in the equation in place of ZT(internal impedance). While this will not be a maximum output circuit,it will be the best working condition for the tube. The rst conjugacycondition for side-tone balance, as given by Equation 1, will bemaintained.

If the tube impedance were actually 10,000 ohms and delivered maximumpower by Equation 3, while Equation 1 held, and if the second conjugacycondition held as given by Equation 2, then it can be shown that thenetwork is a maximum output network, and an E. M. F. at ZL will deliverits maximum power to ZT and ZR while no power is given to ZN from ZL.The total power delivered from ZL is divided between ZT and Za by therelation 4, and/Za will receive all of the power that can be given to itfrom ZL with the chosen value of Y.

From the first conjugacy condition and the reciprocitytheorem, it can beshown that the receiving eiciency, measured by the value of the currentin ZR with an E. M. F. in ZL, is not affected by changes in theresistance of the transmitter. Therefore, if the best value of ZR iscalculated when the network is treated as if it were of the maximumoutput type, with the receiving power the highest that can be given toit with the chosen value of Y, the receiving efficiency will still behighest when the tube impedance is 120,000 ohms. Of course, thisdestroys the second conjugaey condition and the condition that the totalpower delivered from ZL is maximum, but the power transferred into ZR isnot lessened and is still equal to the full amount that it would be ifthe network were of the maximum output type.

The foregoing conclusion may be summed up mathematically by eliminatingZA and ZT from Equations 2 and 3, for the receiving eiiciency isindependent of ZT and ZA.

Rearranging (2) gives Zim/202D Z T Z R and rearranging (3) givesEquating the right hand members of (5) and (6) and solving for Za yieldsHaving chosen a value of Y and xing ZN by Equation 4, Equation 7 thengives the value of ZR for delivery to ZR from ZL of the full receivingpower called for in the ratio Y. When Y vanshes all the power that canbe transferred from ZL is delivered to ZR, but this would leave nopossibility of power delivery to ZL from ZT. With a value Y=.52 used inthe present arrangement, ZR receives from ZL power which is 1.82decibels below all the power that could be transferred from ZL, and thepower delivered into ZL is 4.65 decibels below the maximum that could betransferred from ZT. A gain in receiving of 2.8 decibels, which equalsthe loss in transmitting, is calculated in relation to an induction coilof the conventional type with a value of Y=1.9.

In determining the value of the receiver impedance, as previouslydiscussed, it has also been found necessary to decide where the peakresponse of the reeciving response-frequency characteristic curve shouldbe. A response-frequency characteristic which favors the lowerfrequencies,

will suffer less in effective transmission than a the higherfrequencies. JLIn the latter case, Athe 1" is placed Ori thehieherfrequenies'to. y the' intelligibility,'but` these frequencies are auated the most on the lQnger lines.""I'n accordance with the presentinvention I 'preferfthat the relative'response on the'lowervfrequencies, for xainpl'e'in the range' from approXiinatelySO() to 1,500cycles be increased over that of the' conventional substation set. f"The substation circuit arrangement, as disclosed in Fig. 1, is poweredexclusively from the commercial lighting outlet' at the subscriberssubstation. "Where the commercial lighting source is subject tooccasional failure, itv may be desirable to`pr`ovide a carbonmicrophone" 85 in standby relation to the crystal microphone 5,the'carbon microphone being powered from the battery at the' telephoneexchange. Such an arrangement is illustrated in Fig. 5 wherein thetwo-stage ainplifie'i', including the tubes 2l) and 2t,l is energi'zedfrom the power supply device including Athe rectifiers 36 and 31 withtheir'related filters Fi and F2, asabove described. However, the Vpowersupply includes a relay 89 which is energized as long as the powersupply is functioning to supply the' amplifier. While relay 89is'energiaed, lit opens the connection of the carbon microphone 85'inseries with the telephone line and the bat- ,tory at the telephoneexchange, not shown. When the lighting source fails relay 89 releases tccomplete the connection of the carbon microphone in series with thementioned battery. Under such conditions the carbon microphone 85 isubsiituted. ier the @afstel microg one. i

1. In a telephone subset network of the type comprising""`a` receiver",`telpho'riically" connected thercin for reception of signals from atelephone lineja' microphone connected in said network', an amplifierfor amplifying the response of said microphone and for delivering saidamplified l:responseto the telephone line, said amplier includinganelectron discharge tube having "a filamentary' cathode ui'rectiyheatedby the' 'fie-wbr current thr'ethroughjthe combination therewith of meansfor temporarily causing pulsating current to flow through said cathodewhen the amplifier is initially placed in operation and means forthereafter causing substantially continuous current to flow through saidcathode during the remainder of the operation of the amplifier.

2. In a telephone subset network of the type comprising a receivertelephonically connected therein for reception of signals from atelephone line, a microphone connected in said network, an amplifier foramplifying the response of said microphone and for delivering saidamplified response to the telephone line, said amplifier including anelectron discharge tube having a filamentary cathode directly heated bythe flow of current therethrough, the combination therewith of means fortemporarily causing pulsating current and a surge current to flowthrough said cathode, and means effective shortly thereafter for causingsubstantially continuous current to ow through said cathode during theremaining operation of the amplifier.

3. In a telephone subset network of the type comprising a receivertelephonically connected therein for reception of signals fro-m atelephone line, a mircophone connected in said network, an amplifier foramplifying the response of said microphone and for delivering saidamplified response to the telephone line, said amplifier including anelectron discharge tube having a filain hentay caiiiofie directif heatedby a@ now, @i current therethrough, "thefcmbinati'on therewith of. meansfor temporarily causing unnltered rectified current to flowthrough saidcathodeand means effective shortly thereafter "for causing 'filteredrectified current to lflow through said cathode during the remainingoperation of the ampli er.. 4. A telephonesubset network comprising areceiver telephonically connected therein for the reception of signalsfrom a; telephone line, a microphone connected in said network, anamplifier Afor amplifying the response of said microphone and fordelivering said amplified response to the telephone line, said amplifierincluding an elecytron discharge tube having a filamentary cathodedirectly `heated by the flow ofcurrent therethrough, a'source ofalternating current, a rectifier for converting said alternating currentinto pulsating current, a filter operably connectible to said rectifierfor sultistantiallyv eliminating any alternating current component fromsaid pulsating current whereby substantially continuous current issupplied to said filament, meansfor initially connecting said pulsatingcurrent directly to said filament when the ampliner iis first 'set into'operationjand means effective shortly thereafter to operatively connectsaidffilter to said rectier whereby continuous current is supplied tos'aid cathode. y

5. A telephone subset network comprising a receiver t'elephonicallyconnected thereinv for the reception of signals from a telephone line, ami'- crophone connected in said network, an amplifier-'for amplifyingtheresponse of said microphone and for delivering said amplifiedresponse to the telepl'ione'line,I ysaidfainpliner including anelectrorf ldischarge tube having av filamentary cathode direc'tlyheatedby the flowy of ycurrent therethrough, a source of "alternating current,a rectifier 'for converting said alternating current into pulsatingcurrent, a filter operably connectible to said rectifier forsubstantially eliminating-'any alternating current component from saidpulsating current whereby substantially continuous current is suppliedto said lament, a hookswitch` having a primary set of contact springsand a secondary set of contact springs operating in the sequence named,means including said primary set of contact springs for initiallyconnecting said pulsating current directly to said filament when theamplifier is first set into operation, and means including saidsecondary set of contact springs effective shortly thereafter tooperatively connect said filter to said rectifier whereby continuouscurrent is supplied to said cathode.

6. A telephone subset network comprising a receiver telephonicallyconnected therein for the reception of signals from a telephone line, amicrophone connected in said network, an amplifiel` for amplifying theresponse of said microphone and for delivering said amplified responseto the telephone line, said amplifier including an electron dischargetube having a lamentary cathode directly heated by the flow of currenttherethrough, a source of alternating current, a rectifier forconverting said alternating current into pulsating current, a lter ofthe resistancecapacitor type operably connectible to said rectifier forsubstantially eliminating any alternating current component from saidpulsating current whereby substantially continuous current is suppliedto said filament, means for initially conl1 necting said pulsatingcurrent directly to said filament when the amplifier is rst set intooperation, and means effective shortly thereafter to operatively connectsaid filter to said rectifier whereby continuous current is supplied tosaid cathode.

7. A telephone subset network comprising a receiver telephonicallyconnected therein for the reception of signals from a telephone line, amicrophone connected in said network, an amplier for amplifying theresponse of said microphone and for delivering said amplified responseto the telephone line, said amplifier including an electron dischargetube having a lamentary cathode directly heated by the ow of currenttherethrough, a source of alternating current, a rectifier forconverting said alternating current into pulsating current, a lteroperably connectible to said rectifier for substantially eliminating anyalternating current component from said pulsating current wherebysubstantially continuous current is supplied to said filament, means forinitially connecting said pulsating current directly to said filamentwhen the ampliiier is rst set into operation, and means including adashpot eiective shortly thereafter to operatively connect said filterto said rectifier whereby con tinuous current is supplied to saidcathode.

8. A telephone subset network comprising a receiver telephonicallyconnected therein for the reception of signals from a telephone line, amicrophone connected in said network, an amplifier for amplifying theresponse of said microphone and for delivering said amplied response tothe telephone line, said amplifier including an electron discharge tubehaving ka lilamentary cathode directly heated by the nolw of currenttherethrough, a source of alternating current, a rectifier forconverting said alternating current into pulsating current, a filteroperably connectible to said rectier for substantially eliminating anyalternating current component from said pulsating current wherebysubstantially continuous current is supplied to said filament, ahookswitch having a primary set of contact springs and a secondary setof contact springs operating in the sequence named, means including saidprimary set of contact springs for initially connecting said pulsatingcurrent directly to said filament when the amplifier is first set intooperation, means including said secondary set of contact springseffective shortly thereafter to operatively connect said filter to saidrectifier whereby continuous current is supplied to said cathode, and adashpot controlling the speed of operation of said secondary set ofcontact springs.

9. In a telephone subset network of the type comprising a receivertelephonically connected therein for reception of signals from atelephone line, a microphone connected in said network, an amplifier foramplifying the response of said microphone and for delivering saidamplied response to the telephone line, said amplifier including anelectron discharge tube having a directly heated filamentary cathode, asource of rectified pulsating current for heating said lamentarycathode, and a filter for converting said rectified pulsating currentinto substantially constant current, the combination of means fortemporarily lowering the effectiveness of said filter, whereby theamount of voltage initially available at the output of said iilter israised, and means for thereafter restoring said filter to fulleiectiveness.

ARNOLD LESTI.

REFERENCES CITED The following references are Aof record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,018,464 Nebel Oct. 22, 19352,055,921 Baker Sept. 29, 1936 2,275,581 Barton Mar. 10, 1942 2,282,405Herrick May 12, 1942 2,346,504 Place Apr. 11, 1944 2,369,352 HerrickFeb.` 13, 1945 Certificate of Correction Patent N o. 2,487,455 November8, 1949 ARNOLD LESTI It is hereby certied that errors appear in theprinted specification of the above numbered patent requiring correctionas follows:

Column 7, line 50, for the word ration read ratio; column 8, lines 44and 45, for that portion of the equation reading ZA Z A j; read Z;

and that the said Letters Patent should be read with these correctionstherein that the same may conform to the record of the case in thePatent Ofce.

Signed and sealed this 4th day of July, A. D. 1950.

THOMAS F. MURPHY,

Assistant Oonmssz'olner of Patenls.

